Various exhaust after-treatment devices, such as particulate filters and other devices, have been developed to effectively limit exhaust emissions from internal combustion engines. In the case of compression-ignition or diesel engines, a great deal of effort continues to be expended to develop practical and efficient devices and methods to reduce emissions of largely carbonaceous particulates otherwise present in the engine's exhaust gas.
An after-treatment (AT) system for a modern diesel engine exhaust typically incorporates a diesel oxidation catalyst (DOC) as one of the devices for such a purpose. A DOC generally contains precious metals, such as platinum and/or palladium, which serve as catalysts to oxidize hydrocarbons and carbon monoxide present in the exhaust flow into carbon dioxide and water. The DOC may be used to convert nitrogen monoxide (NO) that is emitted in the engine's exhaust gas stream into nitrogen dioxide (NO2).
An exhaust AT system may also employ a chemical process known as selective catalytic reduction (SCR) for converting oxides of nitrogen (NOX) with the aid of the NO2 generated by the DOC, a catalyst, and a reductant into diatomic nitrogen (N2) and water (H2O). The reductant, which may be an aqueous solution of urea, is generally injected into the engine's exhaust stream during engine operation in proportion to consumption of the engine's fuel. Once in the exhaust stream, the reductant is absorbed onto the system's SCR catalyst where the conversion of NOX takes place.